Field of the Invention
The present invention relates to a touch sensing system with a temperature compensation function.
Discussion of the Related Art
User interfaces (UI) enable humans (users) to interact with various types of electric or electronic devices so that they can easily control the devices as they want. Typical examples of the user interfaces include keypads, keyboards, mice, on-screen displays (OSD), and remote controllers with an infrared communication capability or radio frequency (RF) communication capability. The user interface technology is continuing to make progress toward higher user sensitivity and ease of operation. Recently, user interfaces have been evolving into touch UI, voice recognition UI, 3D UI, etc.
Capacitive touchscreens can be implemented as capacitance sensors. The capacitance sensors may be classified into self-capacitance sensors and mutual capacitance sensors.
As shown in FIG. 1, a mutual capacitance sensor includes mutual capacitance Cm formed between two electrodes Tx and Rx. A sensing part 12 applies a driving signal (or stimulus signal) to Tx lines Tx1 to Tx5, and senses touch input based on a change in the amount of charge charged in the mutual capacitance before and after a touch on Rx lines Rx1 to Rx6. The mutual capacitance Cm decrease when a conductive object is brought closer to it. The sensing part 12 converts the change in the amount of charge to digital data (hereinafter, referred to as ‘touch raw data’) by an analog-to-digital converter and outputs it.
As shown in FIG. 2, a self-capacitance sensor includes self-capacitance Cs formed in each sensor electrode. A sensing part 14 supplies charge to each sensor electrode and senses touch input based on a change in the amount of charge in the self-capacitance Cs. The self-capacitance increases when a conductive object is brought closer to it. The sensing part 14 converts the change in the amount of charge to touch raw data by an ADC and outputs it.
The capacitive touchscreens, if embedded in in-cell type in a pixel array of a display panel, may have poor touch sensitivity because the sensor's capacitance changes with temperature, even without touch input. For example, as shown in FIG. 3, the mutual capacitance Cm changes with temperature with a positive slope, due to a change in capacitance between metal wires of a pixel array. As shown in FIG. 4, the self-capacitance changes with temperature with a negative slope, due to a change in the capacitance of liquid crystals and the parasitic capacitance of TFTs (thin film transistors). A test result shows that there is no change in mutual capacitance at an ambient temperature of 25 if there is no touch input.
Since the amount of charge entering the sensing part 12 or 14 changes with temperature, the value of touch raw data output from the sensing part 12 or 14 changes. In a touch sensing algorithm, touch raw data is compared with a preset threshold to detect the presence or absence of a touch and calculate touch input coordinates. As such, a steep change in temperature may cause a touch input detection error. For temperature compensation, the capacitance of a capacitor Cfb of a pre-amplifier installed at an input terminal of the sensing part 12 or 14 may be increased, or the input range of the ADC may be increased. However, this may cause other problems like an increase in the area of the capacitor and an increase in the power consumption of the ADC.